Typically subsurface safety valves use a pivoting valve member biased by a torsion spring on a pivot. The valve member is known as a flapper and is movable by a tube called a flow tube that is actuated to move by a control system that involves one or more control lines running to the body of the valve and outside the production tubing 5 where the valve is mounted. Surface pressure applied in the control line moves an operating piston which is connected to the flow tube. When the flow tube moves down, the flapper is rotated open and the flow tube advances past it to allow flow through the valve body. Upon release or loss of control line pressure a return spring that acts on the flow tube overcomes the control line hydrostatic pressure and forces the flow tube back up to allow the torsion spring to turn the flapper against a seat to keep production fluids from coming through the valve from below.
In the past redundant control line systems have been provided to secrete operating pistons with both operating pistons connected to the flow tube. The rationale was that if one system failed the other would be available to take over and still operate the valve. With two operating pistons each having one end exposed to hydrostatic pressure in its respective control line and both pistons tied into the same flow tube, the hydrostatic pressure acting on the flow tube was additive of the individual hydrostatic pressures in each of the control lines if both control lines are open. This would mean that the size of the return spring would have to take into account the total hydrostatic pressure from both control lines at any time. One way this was addressed before was to use discrete pressurized gas chambers with one exposed to the back side of each of the pistons and the total force they collectively generated was in excess of the combined hydrostatic pressure from multiple control lines.
Another approach to temporarily isolate backup control lines was to put a rupture disc in the backup line to isolate the hydrostatic pressure in that one line from the flow tube and the spring that would ultimately have to close the valve by overcoming hydrostatic pressure in the control lines. With the backup line closed off with a rupture disc the hydrostatic above it did not affect the workings of the valve and the closure spring acting on the flow tube could be sized for the hydrostatic from a single line. There were two main problems with this design. One was that the specific pressure at which the rupture disc will break is not known. The higher the break pressure the wider the range of pressures specified by the rupture disc manufacturer as to when the disc would break. Another issue was that when a disc would break by design it would not always simply split into fragments that remained attached to the disc assembly. At times fragments would break loose and interfere with the operation of downhole components sometimes rendering them inoperable.
The present invention provides a valve to isolate a control line until it is ready for use. It relies on springs whose force is a more reliable quantity than a break pressure on a complex structure such as that of a rupture disc. The valve is initially closed until application of a predictable pressure moves it to the open position. A locking feature can then hold it in the open position. These and other aspects of the present invention will be more apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings with the understanding that the full scope of the invention is measured by the claims.